1. Field of the Invention
The present invention relates to an arrangement for optical imaging of two subjects located in different object planes onto an optical sensor wherein at least one objective is adjustable relative to the object planes with the assistance of a positioning drive.
2. Description of the Prior Art
In semiconductor technology, superfine structures are transferred onto semiconductor wafers with the assistance of masks, whereby a highly accurate adjustment of the mask relative to the wafer is required. Therefore, for example, the transfer of structures in the sub-micrometer range by means of x-ray lithography requires an adjustment of a mask relative to the wafer with accuracies in the region of below 0.1 .mu.m.
The German Pat. No. 28 22 269 discloses a method for automatic mask adjustment in which the adjustment of successive masks is respectively undertaken with adjustment structures extending orthogonally relative to one another on the semiconductor wafer coated with a photoresist and on the mask. The adjustment structure of the semiconductor wafer is already generated, for example by etching, on the surface of the semiconductor wafer in a first structuring operation, so that the adjustment structures present on the corresponding mask can be aligned with reference to this first adjustment structure in all following structuring operations. For automatic mask adjustment, the adjustment structures on the mask and on the wafer are then scanned with the assistance of an optical sensor in directions extending parallel to their edges, whereupon the intensity of the picture signals generated by the opto-electronic scanning is integrated up line-by-line, or in line sections, and the analog integral values resulting therefrom are converted into digital grey levels and are stored. By forming the difference of the grey levels of successive lines and further evaluation of the resulting difference signals, the position of the middle axis of the adjustment structures can then be acquired, whereupon the semiconductor and the mask are then displaced and turned relative to one another in accordance with the identified offset of the center lines.
In the known mask adjustment on the basis of pattern recognition and image processing at suitable adjustment structures, however, the proximity spacing between the masks and the semiconductor wafer limits the obtainable recognition and adjustment precision. For example, when in a microscopic imaging the focus plane is placed between the mask plane and the wafer plane, then the adjustment structures of both planes are only reproduced in an unsharp manner on the optical sensor. The evaluation of the opto-electronic edge signals which are thereby spread is then not unequivocal for the identification of the relative portion of the mask and the semiconductor wafer, i.e. the accuracy of the position recognition is limited.
In a known apparatus for optical imaging of the adjustment structures of the mask and the semiconductor wafer onto an optical sensor, a two-plane optics is employed wherein either the mask plane or the wafer plane can be sharply imaged onto the optical sensor by two different optical path lengths in the beam path of the imaging optics. Fluctuations in the proximity distance of the mask and the semiconductor wafer, irregularities of the wafer surface and different thicknesses of the photoresist on the wafer surface, however, also lead here to a deterioration of the sharpness of the image of the edge transitions in the individual objective planes. In contrast thereto, the limits of opto-electronic grey scale value image processing can only be achieved when, in the image acquisition, the adjustment structures of the mask and the semiconductor wafer are imaged on the optic sensor with unequivocal sharpness.
The periodical "Siemens Forschungs-und Entwicklungsberichte", Vol. 13, No. 2, 1984, pp. 43-47 already addresses the problems which occur in automatic mask adjustment for x-ray lithography due to fluctuations in the proximity distance of the mask and wafer. These problems should thereby be resolved in that an illumination aperture which is small in comparison to the objective aperture is employed for the optical imaging of the adjustment structure. In accordance with an alternative proposed solution, the objective should be adjusted with the assistance of a piezo-positioning drive such that, first of all, the adjustment structure of the mask and, secondly, the adjustment structure of the semiconductor wafer, are sharply imaged on the optical sensor. It is thereby of decisive significance, however, that the objective must be capable of being adjusted absolutely free of shift.